Measurement of gas partial pressures in physiological fluids by polarography is well-known. Polarographic sensors are used extensively, for example, in the monitoring of pO.sub.2 in blood.
The pO.sub.2 sensors are generally based on a design described by L. C. Clark (e.g. see U.S. Pat. No. 2,913,386) and include a noble metal cathode, a buffered electrolyte and a reference anode. The cathode is normally isolated from the medium under investigation by a permeable membrane, but such a membrane is not essential.
A D.C. potential is applied to electrodes. Oxygen present in the electrolyte (having migrated from the medium under investigation through the permeable membrane or through the skin) is electrochemically reduced at the cathode, and the magnitude of current flow is employed as a measure of pO.sub.2.
The well-known current versus cathode voltage polarogram for pO.sub.2 sensing consists of a curve of increasing current with increasing cathode voltage (to more negative values). The curve has pronounced "knees" at about -600 and -900 mV (all cathode potentials quoted herein are relative to a silver/silver chloride reference anode) with a near horizontal plateau between these values. For pO.sub.2 measurement it is customary to set the cathode polarising voltage on this plateau (typically -750 mV) whereby, as O.sub.2 is reduced at the cathode, current flow is directly proportional to oxygen concentration.
The sensing of pCO.sub.2 in physiological media such as blood is conducted using miniature pH electrodes such as those described by J. W. Severinghaus & A. F. Bradley (1958), J. Appl. Physiol. 13, pp 515-520. Such sensors include a pH electrode (normally a small glass electrode), a reference electrode, and an unbuffered electrolyte. The pH electrode is generally isolated from the medium under investigation by a permeable membrane. Carbon dioxide migrates through the membrane from the medium to dissolve as carbonic acid in the electrolyte. This results in a change in pH which is monitored by the change in EMF between the electrodes. The latter provides a (logarithmic) measurement of pCO.sub.2.
Combined sensors for both pO.sub.2 and pCO.sub.2 measurements have been proposed. One such is described in U.K. Patent Specification No. 2005418 and includes a glass pH electrode for pCO.sub.2 sensing, a silver cathode for pO.sub.2 sensing, a common silver/silver chloride reference electrode and an unbuffered alkaline electrolyte. The components of the sensor were isolated from the medium to be investigated by a permeable membrane. It was somewhat surprising that the pO.sub.2 electrode measurements were unaffected by pCO.sub.2 and vice-versa. Despite the advantages of this combined sensor, it does include glass components (the glass pH electrode) and there may be resistance to its use in in vivo sensing--e.g. intravascularly.
We have now devised an apparatus for simultaneous pO.sub.2 and pCO.sub.2 sensing by polarography and which employs a simple sensor avoiding the use of a separate pH electrode for the pCO.sub.2 measurement.
In an unbuffered electrolyte (i.e. one sensitive to pH changes brought about, say, by changes in pCO.sub.2 certain characteristics of the above-described pO.sub.2 polarogram are pH-sensitive. Not only may these characteristics be employed to measure pO.sub.2, but also pCO.sub.2. This would not have been possible with early designs of pO.sub.2 sensors since the change in pH brought about by the production of hydroxyl ions would itself be significant. However with miniature pO.sub.2 sensors now in use, the current flow on O.sub.2 reduction is so small (measured in nanoamperes) that the corresponding change in the number of hydroxyl ions does not significantly alter the overall pH even in an unbuffered electrolyte.
Thus, based upon these facts, we have now realised that features of a pO.sub.2 polarogram may be employed to measure both pO.sub.2 and pCO.sub.2, thus avoiding the need for a separate pCO.sub.2 sensor.